The fats you eat can dramatically affect your body chemistry. Whether they come from fryer grease or more healthful sources, they pass from your digestive tract into your bloodstream. Traces of these fats end up parking in the cell membranes surrounding each cell of your body. And what they do therewhether they help or hurtdepends on which fats they are.

Omega-3s are often thought of as "good" fats. Their name comes from the fact that the fat molecule is missing a pair of hydrogen atoms at a spot three links from the end (the "omega") of the molecular chain. They have just as many calories as other fats, but they have several beneficial effects on body chemistry.

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First, they are natural anti-inflammatories. Unlike the fats in meat or fryer grease that encourage inflammation, omega-3s act like aspirin or ibuprofen, cooling inflammation. These healthful oils also maintain the fluidity of cell membranes, improving blood flow. In addition, omega-3s help regulate mood. When people run low on omega-3s, they have a tendency toward depression.

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Perhaps their best-known job is to interfere with blood clotting. A blood clot can clog an artery, leading to a heart attack. So, much like aspirin thins the blood, so do omega-3s. However, this is actually a double-edged sword, because there are times when you want your blood to clotwhen you are bleeding, for example. Similarly, an overabundance of omega-3s can increase the risk of hemorrhagic strokebleeding into the brain. So you need some of these useful fats in your diet, but not an excess.

The role of various fats is only now being appreciated. Prior to a report published in 1982, omega-3s were not known to be necessary at all. The case of a young girl changed that: The victim of a gunshot accident, she lost most of her intestinal tract and had to be fed intravenously. As time went on, she developed numbness and blurred vision, and became unable to walk. Her nerve abnormalities turned out to be due to a lack of omega-3s. They were added to her feeding mixture, and her symptoms cleared up.

Q: Fish are often touted as the best source, but I'm not going to eat fish or take fish oil supplements. What should a vegetarian do to get omega-3s?

A: The basic omega-3 is called ALA (alpha-linolenic acid), and the most healthful sources are actually plants, not fish. ALA is in many plants, both on land and in lakes and oceans. You will find traces of it in green vegetables, beans, and fruit, and larger quantities in walnuts, soy products, and especially in ground flaxseeds and flax oil. ALA is the only omega-3 that is essential in the diet. Your body can convert it to other omega-3s: EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). DHA is prevalent in the brain and retina, and may play a role in brain development.

There are several problems with fish and fish oils as an omega-3 source: First of all, a surprising amount of fish fatbetween 15 percent and 30 percentis plain old saturated ("bad") fat, the same sort of fat found in chicken or beef. And, like all animal products, fish have cholesterol. Of course, the best-known problem is contamination, with everything from mercury to PCBs. Because fish are carnivorous, they harbor contaminants from the smaller fish they eat.

Fish get their ALA from plantsjust as humans do. They get it from plankton, and convert it to other omega-3s. As smaller fish are eaten by bigger fish, it then passes up the food chain.

Some have argued that, in humans, ALA is converted into EPA and DHA too slowly, and have suggested supplements as the answer. However, the rate of conversion may depend on how many other fats are competing for the enzymes that lengthen the molecule. So if you are eating lots of grease, it may impair your ability to build the healthful fats you need.

What really matters, though, is not so much the amount of omega-3s in your diet. The key is balance. If someone's diet is loaded with competing fatsfrom hamburgers, cheese, and fried onion ringsthese not-so-healthful fats enter their cell membranes, forcing out the healthful omega-3s. People who take omega-3 supplements may be doing the reverseflooding their bodies with omega-3s to try to force out the "bad" fats. A more sensible approach is to have healthful vegetables, fruits, and beans in your diet. If you like, you can add walnuts, soy products, flax oil, or ground flaxseeds. This gives you the omega-3s you need. At the same time, if you avoid animal products and added oils, the healthful omega-3s can stay in your cells where they belong.

If you are looking to supplement, vegan EPA and DHA supplements, produced from algae, are available. You will find them online and in health food stores.

Where the 3s Are

The Vegetarian Society of the UK recommends vegetarians aim to get 4 grams per day of ALA. Here's where you'll find it.